Mobile station call termination, that is, the ability to detect the presence of an incoming call from the wireless network, is a critical performance issue that is of significant importance to wireless network operators. Typically, the success rate of the mobile station is expected to be very close to 100% under optimum signal conditions, and as high as possible under weak signal conditions.
A failure to properly achieve mobile station call termination can be referred to as a Failure to Terminate (FTT). In most instances a FTT can be caused by the mobile station missing an incoming paging message from the network, or by failing to correctly access the network after receiving an incoming paging message (failure to receive a page), or by a failure to receive a base station acknowledgment (BS—ACK) when attempting to establish a traffic channel in order to handle the call. Of these various failure mechanisms, the failure to receive the page is the most predominant.
In a cellular communication system of most interest to the teachings of this invention, i.e., a Spread Spectrum (SS), Code Division Multiple Access (CDMA) system, the mobile station is enabled to perform soft combining of a traffic channel received from one base station over multiple paths (multi-path), as well to perform soft combining of more than one digital traffic channel (DTCH) received from two or more base stations. This latter technique is typically used during a soft handoff from one base station to the other while the mobile station transitions from the coverage area of one to the other. However, this type of conventional soft combining is not possible when the mobile station is instead receiving and monitoring a digital paging channel (DPCH), as there is, at any given time, only one base station defined in the active base station set. As an example, when a slot cycle index of 1 is used, from slot to slot there is a time separation of 2.56 seconds. During this period of time the signal received from the base station in the active set can change dramatically as a function of at least the mobile station speed and the presence of signal-attenuating obstructions. For example, at slot N, if one assumes that the active pilot channel signal strength is −15 dB, then at slot N+1 (2.56 seconds later) the active pilot signal strength can be reduced to, for example, −19 dB, a figure that is sufficient for the mobile station to re-acquire the system time, but insufficient to enable reliable decoding of the paging channel during slot N+1. The end result is that a page sent to the mobile station can be missed, resulting in the occurrence of the FTT.
It should be noted that at slot N+1 there may be a neighbor base station that has adequate signal strength, and whose paging channel contains the page to the mobile station; however this paging channel is not being monitored or decoded by the mobile station.
It is known in the prior art to attempt to alleviate this problem by performing idle handoffs before the paging channel signal becomes too weak to use. However, this approach assumes that the signal strength will not be reduced by too great an amount between slots (i.e., during the 2.56 seconds assuming that the slot cycle index of 1 is in effect). However, when using the slot cycle index 2 this prior art technique also assumes that the received signal strength will not be significantly degraded during the 5.12 seconds between slots, an assumption that is even more difficult to maintain.